Method for the preparation of diborane



' plosive violence.

United States Patent 3,149,919 METHQD FOR THE PREPARATIQN OF DIBORANERobert D. Schultz, East Whittier, and Carl L. Randolph, In, Whittier,Califl, assignors to Aerojet- General Corporation, Azusa, Calih, acorporation of Ohio No Drawing. Filed Mar. 10, 1955, Ser. No. 493,567

15 Claims. (Cl. 23-204) This invention relates to a new and improvedmethod of preparing diborane.

Diborane is an excellent high energy fuel. Unfortunately, however, itsuse has been severely limited due to a lack of a convenient andinexpensive method for preparing it in high yield.

Heretofore, diborane has been prepared by reacting alkali metalborohydrides with hydrogen chloride gas at room temperature or withliquid hydrogen chloride at a temperature of -78 C. These methods arevery slow, requiring reaction times of from 16 to 24 hours. In addition,the separation of diborane from the resulting 70% diborane-30% hydrogenchloride azeotropic mixture is very difficult.

It has long been desired, as a matter of cost, convenience andcommercial feasibility, to produce diborane by using sulfuric acid asthe protolyzing acid, preferably at room temperature. Unfortunately,when alkali-metal borohydrides were reacted with concentrated sulfuricacid instead of diborane, boron oxides were produced with ex- In anattempt to avoid oxidation, dilute sulfuric acid was substituted in thereaction. Although the borohydride-dilute sulfuric acid reaction was notexplosive, hydrolysis occurred producing boric acid instead of diborane.

We have now found that diborane is rapidly and safely produced in goodyield by reacting a metal borohydride with concentrated sulfuric orchlorosulfonic acid or mixtures thereof in the presence of an aromaticacid or anhydride, or mixtures thereof, in accordance with the generalreaction scheme set forth below:

aromatic acid or anhydride wherein M is a metal radical. Due to thenon-volatile character of sulfuric acid, an azeotropic mixture of gas isavoided. Although rapid, the reaction proceeds smoothly and gentlywithout sparking, flaming, or explosions.

As a matter of convenience, alkali and alkaline earth metal borohydridessuch as potassium and lithium borohydride are usually employed asstarting materials. Commercial grade sulfuric acid, ordinarilycontaining from about 95% to about 98% by weight pure hydrogen sulfateand about to about 2% by weight water, can be used as the protolyzingacid. Chlorosulfonic acid decomposes in water and therefore it ispreferred that the system be anhydrous when this acid is employed.

Aromatic acids and anhydrides useful in this invention are: themonocarboxylic aromatic acids, such as benzoic; the toluic, thenaphthoic and the anthroic acids, and their corresponding anhydrides;and the polycarboxylic aromatic acids, such as phthalic, isophthalic,terephthalic, hemimellitic, tr-imellitic and trimesic acids, and theiranhydrides. The aromatic acids can also be produce in situ by addingtheir corresponding esters or salts to the sulfuric acid, whichhydrolyzes these compounds to the original acids and anhydrides. Estersand salts which are particularly useful are: lower alkyl esters, arylesters and the alkali and alkaline earth metal salts of the acids.

In accordance with the present invention, sulfuric or chlorosulfonicacid containing aromatic acid or anhydride,

. fonic acid, heating if necessary to effect solution.

3,149,919 Patented Sept. 22, 1964 usually in an amount of from about 2%to about 10 by weight of the acid, is reacted with a metal borohydrideand diborane gas is evolved. The preferred method of conducting thereaction is to dissolve the aromatic acid or anhydride in theconcentrated sulfuric or chlorosul- The reaction is usually conducted ina reactor equipped with a gas outlet leading to a low temperature trap.The acid solution is placed in the reactor and finely dividedborohydride added with agitation. During the course of the 7 reaction, agentle evolution of hydrogen and diborane gases takes place. These gasesare vented through the gas outlet and collected in the low temperaturetrap. "If desired, the reaction can also be conducted in a vacuum or anitrogen atmosphere as well as in air.

The mechanism by which the aromatic acid or anhydride modifies thesulfuric or chlorosulfonic acid-borohydride reaction has not beendefinitely established. It is believed, however, that the aromatic acidor anhydride reacts with the protolyzing acid to produce derivativeswhich reduce the surface tension of the protolyzing acid and form acoating about the borohydride particles. The react-ants diffuse intothis coating. In addition, bisulfate ions, produced by the dissociationof the protolyzing acid, become concentrated in the coating, thusproviding a buffer in the reaction zone. By diffusion of the reactantsand buflfering of the reaction, the protolysis of the borohydrideyielding diborane takes place without accompanying oxidation, whichproduces boron oxides, or hydrolysis, which produces boric acid.

To more clearly illustrate this invention, the following examples arepresented. It is to be understood, however, that these examples arepresented merely as a means of illustration and are not intended tolimit the scope of the invention in any way.

EXAMPLE I Benzoic Acid Moderator Benzoic acid in an amount of 3% byweight of the acid mixture was dissolved in sulfuric acid having apurity of 90 cc. of the acid mixture was placed in a 3-necked flaskfitted with a wire stirrer, a gas outlet leading to a low temperaturetrap and a dropping funnel. The reactor was swept out with nitrogen anda nitrogen gas fiow was maintained. Over a period of about 20 minutes,

- 1.5 gm. of sodium borohydride was dropped into the acid mixture.During this time, the temperature was maint-ained at 24 C. and thestirrer was operated at about 300 r.p.m. The reaction proceeded smoothlywithout sparking, flaming or any explosion taking place. Diborane wasrecovered in a yield.

EXAMPLE II Benzoic Anhydride Moderator The reaction was carried out bydropping 8-mesh crystals of sodium borohydride into a test tube holdinga 1 cc. sample of commercial grade concentrated sulfuric acid containingbenzoic anhydride in an amount of 3% by weight of the acid mixture.There was a gentle evolution of gas, which was identified as diborane byits infrared spectrum.

EXAMPLE III 3 EXAMPLE IV Phenylacetic Acid Moderator The reaction wascarried out by dropping 8-mesh crystals of sodium borohydride into atest tube holding a 1 cc. sample of commercial grade concentratedsulfuric acid containing phenylacetic acid in an amount of 1% by weightof the acid mixture. There was a gentle evolution of gas, which wasidentified as diborane by its infrared spectrum.

EXAMPLE V Naphthoic Acid Moderator The reaction was carried out bydropping S-mesh crystals of sodium borohydride into a test tube holdinga 1 cc. sample of commercial grade concentrated sulfuric acid containingnaphthoic acid in an amount of by weight of the acid mixture. There wasa gentle evolution of gas, which was identified as diborane by itsinfrared spectrum.

Optimum results are obtained by first dissolving the modifier in theprotolyzing acid and subsequently adding the metal borohydride to thesolution. This procedure provides a homogeneous reaction mixture inwhich maximum protection of the metal borohydride particles from theoxidizing influence of the protolyzing acid is obtained as well asmaximum surface area for reaction. It is often desirable to heat themodifier-protolyzing acid mixture to effect solution, however, thetemperature at which the diborane producing reaction proceeds is notlimited by this expedient. The reactants and product of the reaction arethermally stable and the temperature at which the reaction is conductedis not critical, although as a matter of convenience the reaction isconducted at about room temperature.

We have invented a means of moderating the reaction of borohydrides withsulfuric or chlorosulfonic acid so that diborane is produced safely,rapidly and conveniently. Due to the inexpensiveness and availability ofsulfuric acid, ease of running the reaction at ordinary temperatures,and the convenience of recovering diborane from the reaction mixture,the above described method of preparing diborane will find valuable usein the production of this high energy fuel. As well as being useful as ahigh energy fuel, diborane also finds valuable use in vulcanizingrubber, as disclosed in United States Patent No. 2,558,559.

We claim:

1. A method of producing diborane which comprises reacting a protolyzingacid selected from the group consisting of concentrated sulfuric andconcentrated chlorosulfonic acid and mixtures thereof with a metalborohydride selected from the group consisting of the alkali andalkaline earth metal borohydrides in the presence of a reactionmoderator selected from the group consisting of aromatic carboxylicacids, anhydrides and mixtures thereof.

2. The method of claim 1 wherein the said reaction moderator is presentin an amount of from about 2% to about 10% by weight of the protolyzingacid.

3. The method of claim 1 wherein the said reaction moderator is benzoicacid.

4. The method of claim .1 wherein the said reaction moderator is benzoicanhydride.

5. The method of claim 1 wherein the said reaction moderator is phthalicanhydride.

6. The method of claim 1 wherein the said reaction moderator isphenylacetic acid.

7. The method of claim 1 wherein the said reaction moderator isnaphthoic acid.

8. The method of claim 1 wherein said protolyzing acid and said reactionmoderator are first mixed together and said metal borohydride then addedto the mixture.

9. The method of claim 1 wherein the metal borohydride is sodiumborohydride.

10. The method of claim 1 wherein the metal borohydride is potassiumborohydride.

11. The method of claim 1 wherein the metal borohydride is lithiumborohydride.

12. A method of preparing diborane which comprises reacting sodiumborohydride with concentrated sulfuric acid in the presence of fromabout 2% to about 10% benzoic acid, by weight of the sulfuric acid.

13. A method of preparing diborane which comprises reacting sodiumborohydride with concentrated sulfuric acid in the presence of fromabout 2% to about 10% phthalic anhydride, by weight of the sulfuricacid.

14. A method of preparing diborane which comprises reacting sodiumborohydride with concentrated sulfuric acid in the presence of fromabout 2% to about 10% phenylacetic acid, by weight of the sulfuric acid.

15. A method of preparing diborane which comprises reacting sodiumborohydride with concentrated sulfuric acid in the presence of fromabout 2% to about 10% naphthoic acid, by weight of the sulfuric acid.

References Cited in the file of this patent UNITED STATES PATENTS GibbJuly 4, 1950 Chiras Mar. 31, 1959 OTHER REFERENCES

1. A METHOD OF PRODUCING DIBORANE WHICH COMPRISES REACTING A PROTOLYZINGACID SELECTED FROM THE GROUP CONSISTING OF CONCENTRATED SULFURIC ANDCONCENTRATED CHLOROSULFONIC ACID AND MIXTURES THEREOF WITH A METALBOROHYDRIDE SELECTED FROM THE GROUP CONSISTING OF THE ALKALI ANDALKALINE EARTH METAL BOROHYDRIDES IN THE PRESENCE OF A REACTIONMODERATOR SELECTED FROM THE GROUP CONSISTING OF AROMATIC CARBOXYLICACIDS, ANHYDRIDES AND MIXTURES THEREOF.